Router Ranker

NAS Charts

NAS Ranker

More Tools

LAN & WAN How To

Broadcasts

VLANs become important when you consider broadcasts. Broadcasts
are frames sent to all devices on a switch, and in many cases, a normal and
frequent function. A broadcast domain is the set of all devices that receive a
broadcast. Small LANs are typically equivalent to a single broadcast domain.

Devices on a network generate significant broadcast traffic.
Broadcasts normally occur when a device is trying to send data to another
device, but doesn't know the MAC address of the destination device. A PC that knows the
destination IP, but not the MAC associated with that IP, will send a broadcast.
This type of broadcast is an ARP (Address Resolution Protocol) broadcast.

Devices, such as PCs, will build and maintain a listing of
IP addresses to MAC addresses in what is known as the ARP cache. The ARP cache
is temporary, can be overwritten, and is rebuilt every time the PC is
powered on. In addition, entries expire after two minutes on Windows XP and 2000 PCs.

In a Windows PC, you can see the ARP cache by typing arp -a
at the command line. In Figure 2, you can see the ARP cache of my PC, as learned from
its network interface.

Figure 2: arp -a command showing ARP cache

Another example of broadcasts generated by PCs is DHCP
(Dynamic Host Configuration Protocol) requests. PCs will send DHCP broadcast
requests when they're turned on to acquire an IP address, unless their IP has
been statically configured.

Another source of broadcasts are switches themselves. When a
frame enters a switch destined for a MAC address that the switch hasn't learned—and that thus isn't in the switch's MAC table—the switch will broadcast that frame
to all devices except the one that sent it, looking for a response.

The device with the desired MAC will respond to this
broadcast. The switch will then update its MAC table with what it learned from
the port on which the response frame was received. Like a PC, the MAC table of
a switch is usually stored in temporary memory, and will be rebuilt every time the switch
is powered on.

IP multicasts are yet another source of broadcasts. Video can be
sent over IP multicasts, which can consume tremendous amounts of bandwidth. For this reason, IP multicasting is frequently disabled in large networks and in most consumer routers by default.

Broadcasts can eat up considerable bandwidth on your LAN and they
also use processing power. Every device in the LAN receives broadcasts and must
read and determine whether or not to respond to each broadcast. As the number of devices in your LAN grows, so will the
volume of broadcast traffic.

This is where VLANs become valuable—to break
up broadcast domains. Broadcasts are propagated within a VLAN, but not between
VLANs. By segmenting a network into VLANs, you will increase usable network
bandwidth, resources, and performance through the reduction of broadcast
traffic.

Routers

Routers also break up broadcast domains. Routers operate at Layer 3, forwarding
packets based on IP addresses, not MAC addresses. A router will receive a frame
on its Ethernet interface, strip off the MAC address, and make a routing
decision based on the originating and destination IP addresses.

Routing is an integral part of any network that contains multiple subnets and can play a key
part in VLANs. VLANs can be configured on separate subnets, requiring a
router to provide access to common services required by each VLAN.

For example, a network connected to the Internet usually
employs a gateway router, which is probably also providing DHCP and NAT (Network
Address Translation) services. If VLANs are created on different subnets, then the gateway, or another router will need to provide those services to each VLAN. In larger LANs, inter-subnet routing and VLAN segmentation is often handled by Layer 3 (sometimes called "multilayer") switches.

VLANs can also be configured to share a
single subnet, yet isolate various LAN members from each other. I'm going with the
single subnet approach here, using the SRW as my Layer 2 managed switch and a
Linksys RV042 router (Figure 3) for Internet access, DHCP, and NAT.